Known control strategies for point feeding are based in principle on an ideal correlation between the cell resistance and the oxide concentration which is represented by a U-shaped curve in which the concentration is given along the x axis and the cell resistance along the y axis. See FIG. 1, top curve. The base of the curve will usually lie at 4% oxide concentration. Such a control strategy involves controlling the cell by keeping the concentration low, below 4%. In this area, the cell resistance will increase as the concentration decreases while it will decrease as the concentration increases. In practice, this can done in such a way that, if the cell resistance is measured, for example, at a value corresponding to an oxide concentration of 2%, the cell can be overfed with oxide so that the oxide concentration in the bath increases. After a period of time, typically 30-60 minutes, the oxide concentration will typically be approximately 3% and the cell resistance will have decreased. The cell will subsequently be underfed with oxide so that the concentration falls again. When the oxide concentration is 2% again, a new overfeeding period can be started. Using this control strategy, the cell is excited in respect of the oxide in order to obtain a signal in the cell resistance which is used to control the cell's oxide concentration in the bath at a relatively low level. The term "R signal" will be used in the following as a designation of the cell resistance in the periods surrounding a change in feed rate. In the following, the term "rapid feeding" represents a fixed overfeed rate.
______________________________________ Definitions: ______________________________________ U Period of time with underfeeding R Period of time with rapid feeding UH period Period of time with U + H H on Start time for rapid feeding U on Start time for underfeeding ______________________________________
A so-called anode effect occurs if the oxide concentration in the bath becomes low enough (approximately 1.8%). In connection with the anode effect, it is normal for the voltage immediately to rise to approximately 50 volts. The effect normally lasts approximately 5 minutes. Special measures are usually necessary to remove the anode effect. Anode effects may be desired or undesired. One advantage of the point feeding technology is that the frequency of anode effects can be reduced radically.
The energy supply to an electrolysis cell can be controlled by adjusting the anode up and down in the periods without an anode effect. Automatic adjustment is based on measurement of the cell's ohmic resistance. If the measurement is outside a deadband around a resistance reference, the anode is adjusted. An upper and a lower deadband are used. The two deadbands and the reference can vary automatically depending on the state of the cell.
In order to optimise the operating conditions and to maximise the financial return, it is desirable to keep a low concentration of oxide in the bath. In order to determine the point which corresponds to an oxide concentration of approximately 2%, i.e. a concentration which is normally slightly higher than that which may produce an anode effect in the cell, the level change in the resistance is used, for example, in a period before the point is reached, or the angle coefficient of the resistance near to this point. The angle coefficient of the resistance can be determined on the basis of an equation for a straight line in an x-y co-ordinate system, i.e. y=ax+b, where a is the angle coefficient. A control strategy may be based on a combination of both level change in the resistance and the angle coefficient of the resistance. The decision that the point has been reached is called prediction (i.e. anode effect prediction).
The problem with known prediction methods is that the ideal correlation between the resistance and the concentration on which the methods are based can be seriously disturbed by other conditions in the cell, conditions which affect the development of the resistance over time. Such disturbances are particularly large in S.o slashed.derberg cells and lead to many false predictions. False predictions are predictions which occur at relatively high oxide concentrations in the bath. True predictions are predictions which occur at sufficiently low oxide concentrations in the bath.
Change in resistance on account of the absorption of oxide sludge from the base and side coating is an example of a disturbance. The absorption of sludge causes the metal level to fall with an increase in resistance as a result. The change in resistance per time unit depends on many factors such as the supply of energy to decompose the oxide, the quantity of sludge in the cell, the chemical and mechanical availability of the sludge, the geometry of the side coating, the bath quantity, etc. Mechanical availability in this connection includes the liquid flows in the bath and metal, among other things. The flow paths and physical flow rates are significant.
In all aluminium electrolysis cells, the oxide concentration in the bath will always depend on two oxide sources: oxide supplied to the bath from the outside and oxide supplied to the bath from the inside. Oxide from the inside comes from base sludge and the side coating. In connection with the supply of oxide to the cell through the bath cake at a point feeding point, some oxide may pass through the bath phase without being decomposed. This oxide becomes sludge. The quantity which becomes sludge depends, among other things, on the oxide concentration in the bath and the local supply of energy to the oxide dose. A high concentration and low "overtemperature" favour the formation of sludge. Overtemperature means the difference between the temperature in the bath and the bath's melting point (liquidus temperature). The above two oxide flows can vary greatly in a point-fed S.o slashed.derberg cell. In a point-fed prebake cell, the oxide flow from the feed points normally dominates.
If a cell has a lot of easily available sludge, a high concentration of oxide in the bath can be maintained for a long period through sludge absorption. The sludge which disappears will often lead to an increase in resistance. This increase in resistance often leads to false predictions. The prediction produces a relatively high feed rate of oxide through the feeders for a certain period of time (overfeeding). In this way, oxide is supplied which gradually becomes new sludge. The high concentration of oxide in the bath can last for a long time. Periods of high concentration reduce the financial return.
Other variables in a cell can also lead to false predictions, for example temperature change, change in bath chemistry and change in the shape of the side coating.
Other examples of states in which a known control does not always work expediently are the period after an anode effect, the period after tapping, the period after side or end precipitation, other periods with a high oxide concentration in the bath, periods with an extremely low oxide concentration in the bath, periods with high noise in the cell's resistance, periods with high sludge absorption and periods with a high temperature in the bath.
With reference to the above, known control strategies will be characterized to a greater or lesser degree by false predictions and high oxide concentration because a good enough overview of the oxide concentration is not available at any point in time. S.o slashed.derberg cells are normally more subject to false predictions than prebake cells.
There is, therefore, a great need for a control strategy which can contribute to achieving good control over the oxide concentration so that the predictions made are true.